Typically, vehicles such as automobiles are occasionally installed with a traction control apparatus and the like to prevent drive slip. There has been known that, when an acceleration operation, a low-μ-road driving or the like causes drive slip, such a traction control apparatus performs a braking control of a brake and driving control of an engine to generate appropriate traction on wheels, thereby preventing wheel slip.
When the traction control apparatus is installed in a two-wheel-drive car, a vehicle speed can easily be estimated by detecting rotation speeds of driven wheels (not driving wheels) by a sensor and the like.
However, since all wheels of all-wheel-drive vehicles such as a four-wheel-car are driving wheels, all the wheels may generate drive slip. Accordingly, it is difficult to accurately estimate a vehicle speed only by detecting rotation speeds of all the wheels.
For this reason, there has been proposed a technique for estimating a vehicle speed of such an all-wheel-drive vehicle installed with a rotation speed sensor for wheels and an acceleration sensor, the technique including selecting a select wheel to be referred based on a rotation speed of each of the wheels by the rotation speed sensor and estimating the vehicle speed based on an output from the acceleration sensor (see, for instance, Patent Literature 1).